A Shocking Exploration.

Here's a neat critter from the Amazon:

Electric Eel (Electrophorus electricus),
Shedd Aquarium, Cook Co, IL  1/20/2013

This is an Electric Eel, (Electrophorus electricus). They live in shallow waters in South America, often in ponds and back channels that nearly disappear in the dry season. Oxygen levels can get very low in these waters, and the Electric Eel is not only capable of breathing air through an organ in its mouth, it actually has to, every 10 minutes or so! (1)

Of course, it's called an Electric Eel because it's got a bunch of specialized cells that can generate up to 500 watts of electric current over a very short time. Known as electrocytes, they are modified muscle cells. Where does the electricity come from?

Well, most animals can produce electric currents -- nerve cells conduct information by opening ion channels in their membranes, allowing charged ions (mostly Sodium and Potassium) to move in and out of the cells. Muscle cells use a similar system to signal the actin and myosin proteins to start working to contract the cell. So all the eel had to do is decouple the motor system from the signaling system, and then stack a whole bunch of these cells on top of each other.

Still, that's a fair bit of work for evolution to do without some intermediate payouts. What good is a little bit of electricity?

Turns out, quite a bit. Other members of the same family, as well as several species of African fish, produce weak electric fields that aren't much use for self-defense, but are very good at detecting living critters in the surrounding water. Since they don't require light, these guys can still hunt at night, or in very murky water.

Electric Eels didn't have to invent the system from scratch -- early, weakly electric fish modified an already working system, and then the eels simply strengthened it. This is typical of evolution -- it uses what's handy. Fins become forelimbs, forelimbs become wings, and suddenly there's a whole range of places for evolution to explore.

Something like this may have happened with the electric fish in yet another sense. In most species, predation pressure is considered to oppose sexual selection, such that a male's features (usually the male, anyways) are a balance between attractiveness and survival. But in the Eel's relatives, the signals they use to hunt and to communicate appear to have evolved in such a way as to make them harder to detect by electro-sensitive fish. (Especially the Electric Eel, interestingly enough.) Males of these species now appear to have coopted the new features of those signals for indications of fitness, in much the same way as frogs do with their calls. (2) So we have a case where a feature has evolved a new set of uses from a very different basis, just like the electric signals the fish put out in the first place.

(1) Johansen, Kjell (1968). "Gas Exchange and Control of Breathing in the Electric Eel, Electrophorus electricus".Z. Vergl. Physiologie (Springer Berlin / Heidelberg) (Volume 61, Number 2 / June, 1968): 137–163.

(2) Stoddard, P. K. (1999). Predation enhances complexity in the evolution of electric fish signals. Nature, 400(6741), 254-256.